Carbon dioxide lasers are widely used for surgery, cutting and welding of metals and military applications. The wavelength of a CO.sub.2 laser is in the range of about 9-11 um. While there are currently seven available CO.sub.2 laser frequencies, the most commonly used wavelength is 10.6 um.
Problems have been encountered in producing optical fibers suitable for transmission of light from a CO.sub.2 laser because of difficulties encountered in fabricating a fiber with the necessary characteristics. Currently, it appears that a hollow core fiber may be particularly useful for this purpose. See, for example, Hidaka et al U.S. Pat. No. 4,453,803.
In any optical fiber, the cladding must have an index of refraction lower than that of the core so that total internal reflection occurs between the core and cladding interface to confine the light energy to the core. In a hollow core fiber, the core is air which has an index of refraction of one; therefore, the cladding material of a hollow core fiber for use with a CO.sub.2 laser must have a refractive index less than one in the mid infrared spectral region.
Hidaka et al (supra) have proposed a hollow core optical fiber for CO.sub.2 laser transmission in which the cladding consists of germanium dioxide, zinc oxide and potassium oxide glass. However, conventional methods used to manufacture such glass compositions give rise to problems in maintaining high geometrical tolerances and purity. As a result, hollow waveguides fabricated with this composition by conventional techniques tend to exhibit performance limitations due to these problems. Entrained glass contaminants, usually in the form of air bubbles, coupled with variations in bore dimensions and circularity result in high attenuation and adverse heating effects that can cause thermal failure.
Vapor deposition processes have been developed for the fiberoptic telecommunications industry. These methods involve oxidation of reagent vapors to form a soot product which is subsequently consolidated into a high purity glass. Because the starting reagent materials are in the form of high purity gases or liquids, glass made by vapor deposition methods exhibit extremely high purity levels. Moreover, very high dimensional tolerances can be obtained using known vapor deposition manufacturing technology and equipment.
Despite the benefits of vapor deposition processes, they have a disadvantage in that they are limited in glass composition range. Because the starting materials in a vapor deposition process are transported in a vapor phase before oxidation, the starting materials must generally be volatile at less than 200.degree. C. in order to be processed. Glass compositions of the GeO.sub.2 --ZnO--K.sub.2 O type are very difficult to fabricate by vapor deposition processes because the zinc and potassium components are not readily available in volatile compounds or volatile organo metallic form. Hence, despite the benefits available from vapor deposition processes, such processes are not known to have been used in the manufacture of hollow core optical fibers.
An object of the present invention is to provide a method of manufacturing hollow core optical fibers of very pure glass materials and uniform dimensions.
Another object is to provide a glass composition for use as the cladding material in a hollow core optical fiber wherein the components of such composition are conveniently available in volatile form such that the cladding can be formed by vapor deposition processes.
A further object is to provide novel cladding compositions for a hollow core optical fiber which can be "tuned" by selection of components to various frequencies in the mid infrared spectral range.